The present invention generally relates to a field emission display (FED), and more specifically to a cathode plate of a carbon nano tube (CNT) FED and its fabrication method.
Although liquid crystal display (LCD) has become one of the most popular display devices, many researches on different kinds of display technologies are being pursued. One of them is the field emission display that has the potential to replace the conventional liquid crystal display in the display market. Unlike the conventional cathode ray tube (CRT) that uses a hot cathode electron gun, a field emission display uses cold cathode emitter tips as the electron source. When a field emission display is placed in an electric field, cold cathode emitter tips aim at the phosphor-coated anode substrate that is fabricated in the field emission display and emit a bundle of electrons.
FIG. 1 shows a schematic diagram of a conventional field emission display. Electrons, being attracted by an electric field and emitted out of the cold cathode 102 emitter tip 103 on the cathode glass substrate 101, are accelerated by the positive voltage applied to the anode substrate 104 to hit the coated phosphor 106 on the anode 105 and then produce luminescence.
Most conventional cathode plates of field emission displays are fabricated by a screen printing method. In this method, a pre-mixed paste is applied to the surface of a pre-patterned screen and scraped using a scraper to print the pattern onto a substrate. Such process is repeatedly used to stack layers of patterns. The method has some drawbacks. It is difficult to increase the resolution of the printed pattern because of the limitation of the size of the screen mesh. The initial field emission voltage must be high enough to get sufficient brightness for the display. Also, the thickness of the printing film may not be uniform enough and the printed pattern may be inaccurate due to the non-uniform tension of the screen. Therefore, the distribution of the electric field is non-uniform and the alignment at post-process is difficult.
In order to be accepted in the display market, field emission displays must overcome the above-mentioned drawbacks by exploring a new fabrication method. The new fabrication method has to be capable of increasing the resolution of the printed pattern as well as the uniformity of the thickness of the printing film. It must also be able to arrange patterns arbitrarily to increase the function of the display.
This invention has been made to overcome the above-mentioned drawbacks of conventional field emission displays. The primary object is to provide a fabrication method for the cathode plate of a carbon nano tube field emission display. By combining photolithography process and etching process, the method uses a photosensitive paste and an etchable dielectric material to fabricate the cathode plate of a carbon nano tube field emission display.
According to this invention, the fabrication method for the cathode plate of a carbon nano tube field emission display comprises the preparation of a transparent substrate and the fabrication of a cathode electrode layer, a dielectric layer, a gate layer, and a CNT emission layer. During the fabrication, a transparent substrate having top and bottom surfaces is first prepared. A layer of photosensitive paste is deposited on a surface of the transparent substrate. A pattern is then defined by a photolithography process and sintered to finish a cathode electrode layer.
The whole surface of the cathode electrode layer is deposited with a layer of etchable dielectric material. A layer of photosensitive gate material is further deposited on the dielectric layer. Gate patterns are then printed by a photolithography process and sintered to finish a gate electrode layer. The gate pattern is used as a protecting film to etch a portion of the dielectric layer not covered by the protecting film in a photolithography process and finally a CNT emission layer is filled on the cathode electrode layer to form a cathode plate structure.
In the preferred embodiments of this invention, the transparent substrate, such as a glass substrate, has top and bottom surfaces. The photolithography process includes the definition of a pattern by a photo-mask after pre-bake, photo exposure and development. Before filling the CNT emission layer, the gate pattern is used as a protecting film to etch a portion of the dielectric layer to form a dielectric pattern in a photolithography process. The CNT emission layer can be filled by a photolithography method or an electrical deposition method. In the present invention, following the fabrication of the cathode electrode layer, the CNT emission layer can also be fabricated before the dielectric layer and the gate electrode layer by a screen printing method.
According to this invention, the cathode plate in a carbon nano tube field emission display can increase the resolution of patterns as well as the uniformity of the film thickness. Also, patterns can be arbitrarily arranged. Packing this cathode plate structure and a conventional anode plate together can make a CNT field emission array.
In a preferred embodiment of the invention, the line width of the CNT field emission array can be as small as 30 xcexcm per line space in a photolithography process under the conditions that the exposure energy for the photolithography process is 300-500 mJ, the development is at 20-25xc2x0 C. room temperature, and the weight percentage (wt %) of the K2CO3 solution is 0.5-0.7. Comparing with patterns fabricated by conventional screen printing methods, the resolution of the printed pattern is significantly increased by this invention. The resolution of the dielectric layer can be decided during the fabrication process for the gate electrode layer. This reduces the complicated alignment and simplifies the fabrication process. Therefore, the quality of the display is improved.
The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings.